Add per-photo-widget lock (freezes current photo until unlocked)

A "Lock this photo" button in the photos widget's dialog toggles
PhotoWidgetConfig.locked, which suppresses both the timer-elapsed
auto-advance and the advance/back button actions until unlocked. The
Layout tab canvas shows a lock badge on any locked photo widget's box.

server/tests/test_widgets_photos.py

@@ -109,6 +109,41 @@ def test_advance_action_is_a_no_op_when_unconfigured(db_session):
     assert cfg.current_asset_id == ""
 
 
+def test_render_does_not_advance_when_locked(db_session, monkeypatch):
+    frame, widget = _make_widget(db_session)
+    monkeypatch.setattr(widgets.photos, "immich_client_for", lambda frame: object())
+    monkeypatch.setattr(widgets.photos, "list_assets", lambda client, album_id: _ASSETS)
+    source = Image.new("RGB", (100, 80), (10, 20, 30))
+    monkeypatch.setattr(widgets.photos, "fetch_source_and_faces", lambda client, mode, asset_id: (source, None))
+
+    with widget_locked(db_session, frame.id, widget.id) as (_, _, cfg):
+        cfg.current_asset_id = "asset-1"
+        cfg.current_asset_set_at = 0.0  # long ago -- refresh_interval_s has definitely elapsed
+        cfg.locked = True
+
+    widgets.photos.render(db_session, frame, widget, 400, 300)
+
+    cfg = db_session.get(PhotoWidgetConfig, widget.id)
+    assert cfg.current_asset_id == "asset-1"
+
+
+def test_advance_and_back_actions_are_no_ops_when_locked(db_session, monkeypatch):
+    frame, widget = _make_widget(db_session)
+    monkeypatch.setattr(widgets.photos, "immich_client_for", lambda frame: object())
+    monkeypatch.setattr(widgets.photos, "list_assets", lambda client, album_id: _ASSETS)
+
+    with widget_locked(db_session, frame.id, widget.id) as (_, _, cfg):
+        cfg.current_asset_id = "asset-1"
+        cfg.locked = True
+
+    widgets.photos.ACTIONS["advance"](db_session, frame, widget)
+    widgets.photos.ACTIONS["back"](db_session, frame, widget)
+
+    cfg = db_session.get(PhotoWidgetConfig, widget.id)
+    assert cfg.current_asset_id == "asset-1"
+    assert cfg.history == []
+
+
 def test_advance_uses_the_frame_stats_counter_not_the_widget_config():
     """advance_forced's frame/stats split (see app/photo_queue.py) means
     stats_photos_displayed should land on the Frame row, never on